MODIFIED STRUCTURAL LAYOUTS FOR STAGGERED TRUSS FRAMING SYSTEMS USED IN SEISMICALLY ACTIVE AREAS

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska MODIFIED STRUCTURAL LAYOUTS FOR STAGGERED TRUSS FRAMING SYSTEMS USED IN SEISMICALLY ACTIVE AREAS S. Simasathien 1, S-H. Chao 2, K. Moore 3, and T. Okazaki 4 ABSTRACT The steel Staggered Truss Framing (STF) system is a highly economical and efficient steel framing system that accommodates generic architectural and structural requirements for mid- to high-rise buildings. Due to the attributes of enabling small floor-to-floor height, large columnfree spaces, reduced number of columns, efficient use of material, light weight, reduced size and load requirements for the foundation, high lateral stiffness, rapid construction, and low overall cost, STFs have become a popular system in regions of low seismicity. Although STFs were originally developed for low seismic regions, the high lateral stiffness and light weight make this system attractive for use in high seismic regions. However, very limited studies have been conducted on the behavior of STFs under strong earthquake ground motions. A key to the lateralload resisting mechanism of STFs is the active participation of the floor diaphragms (typically consists of prestressed hollow-core planks) to transfer the inertial forces cumulating in a staggered manner across the height of the structure. The increasingly large diaphragm shear force in the lower stories brings concerns regarding the cyclic behavior of diaphragm-to-truss connections, local stress demand in the diaphragms under in-plane force and out-of-plane displacement. The stability of the system needs to be investigated if the trusses are designed to take inelastic action since the trusses serve as both the gravity and lateral-load resisting systems. In this paper, limitations of the conventional STF for use in seismically active areas are discussed. Subsequently, an alternative structural layout was investigated in which the conventional diaphragms composed of precast concrete panels are replaced by horizontal steel trusses to transmit the large diaphragm shears in the lower stories. A prototype mid-rise STF building was designed according to the spectrum specified in ASCE-7 and the seismic performance of the 3-D model was evaluated by nonlinear time-history analyses. 1 PhD Candidate, Dept. of Civil Engineering, University of Texas at Arlington, Arlington, TX Associate Professor, Dept. of Civil Engineering, University of Texas at Arlington, Arlington, TX Principal, Simpson Gumpertz & Heger Inc., CA Associate Professor, Graduate School of Engineering, Hokkaido University, Hokkaido, Japan Simasathien S, Chao S-H, Moore K, and Okazaki T. Modified structural layouts for staggered truss framing systems used in seismically active areas. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska Modified Structural Layouts for Staggered Truss Framing Systems Used in Seismically Active Areas S. Simasathien 2, S-H. Chao 2, K. Moore 3, and T. Okazaki 4 ABSTRACT The steel Staggered Truss Framing (STF) system is a highly economical and efficient steel framing system that accommodates generic architectural and structural requirements for mid- to high-rise buildings. Due to the attributes of enabling small floor-to-floor height, large column-free spaces, reduced number of columns, efficient use of material, light weight, reduced size and load requirements for the foundation, high lateral stiffness, rapid construction, and low overall cost, STFs have become a popular system in regions of low seismicity. Although STFs were originally developed for low seismic regions, the high lateral stiffness and light weight make this system attractive for use in high seismic regions. However, very limited studies have been conducted on the behavior of STFs under strong earthquake ground motions. A key to the lateral-load resisting mechanism of STFs is the active participation of the floor diaphragms (typically consists of prestressed hollow-core planks) to transfer the inertial forces cumulating in a staggered manner across the height of the structure. The increasingly large diaphragm shear force in the lower stories brings concerns regarding the cyclic behavior of diaphragm-to-truss connections, local stress demand in the diaphragms under in-plane force and out-of-plane displacement. The stability of the system needs to be investigated if the trusses are designed to take inelastic action since the trusses serve as both the gravity and lateral-load resisting systems. In this paper, limitations of the conventional STF for use in seismically active areas are discussed. Subsequently, an alternative structural layout was investigated in which the conventional diaphragms composed of precast concrete panels are replaced by horizontal steel trusses to transmit the large diaphragm shears in the lower stories. A prototype mid-rise STF building was designed according to the spectrum specified in ASCE-7 and the seismic performance of the 3-D model was evaluated by nonlinear time-history analyses. Introduction STF system has been widely used in low-seismic regions because of the apparent benefits of the system. However, there are attributes that limit the use of STF system in areas of moderate to high seismicity such as the stability of gravity system due to the fact that the lateral and gravity force-resisting systems are one and the same and the increasingly large diaphragm shear force in the lower stories. Moreover, the large rotation at the ends of the chord members in the single 1 PhD Candidate, Dept. of Civil Engineering, University of Texas at Arlington, Arlington, TX Associate Professor, Dept. of Civil Engineering, University of Texas at Arlington, Arlington, TX Principal, Simpson Gumpertz & Heger Inc., CA Associate Professor, Graduate School of Engineering, Hokkaido University, Hokkaido, Japan Simasathien S, Chao S-H, Moore K, and Okazaki T. Modified structural layouts for staggered truss framing systems used in seismically active areas. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 Vierendeel panels in the conventional STFs limits the overall drift capacity of the system. This paper presents the design and nonlinear analyses of a prototype STF structure with modified structural layouts utilizing horizontal steel trusses to transfer the in-plane diaphragm forces along with diagonal braces at the non-truss levels to alleviate the demand on columns. Additional braces ( kickers ) are added in the story levels without floor-height trusses in order to prevent large column deformations that occur due to relatively smaller story stiffness as compared to stories with story-height truss. Diagonal web members are also added in the outermost panels of the posts and hangers. In addition, the Vierendeel panels in the center of the trusses are modified to span over three panels of the truss to increase the overall drift capacity of the structure. Unlike the conventional STF structure, the precast planks are arranged in the transverse direction as the horizontal trusses serves as transfer diaphragms. Issues of STFs Subjected to Seismic Loads Limited studies have been conducted on the behavior of STFs under strong earthquake ground motions particularly on the high in-plane forces in the precast hollow-core slab diaphragm typically utilized in STFs. To the authors best knowledge; there is no test data available on this subject. The shear force transfer between the precast hollow-core slab and steel truss in the exterior bay and the increasingly large diaphragm shear force in the lower stories, as shown in Fig. 1, brings concerns regarding the cyclic behavior of diaphragm-to-truss connections. Due to rigidity of the connections, large moment would develop in the diaphragm-to-truss connections when the structure displaces along the longitudinal direction. The connections can be subjected to bi-axial bending. Large openings on the floor, e.g. stair cases or elevators, also affect the shear transfer across the diaphragms. (a) (b) Figure 1. Diaphragm-to-truss connections: (a) interior diaphragm; (b) exterior diaphragm. Another issue with the STFs subjected to seismic loads is that the lateral force-resisting frames are the same as the gravity systems in the transverse direction. The stability of gravity systems is compromised if the seismic-force-resisting members yield due to the large rotation demand at the ends of the chord members in the single Vierendeel panels in the conventional STFs. The large rotation of the chord members also limits the overall drift capacity of the system. Eq. 1 shows the approximate relationship between the story drift ratio and plastic hinge rotation of the chord members. For typical STFs with the ratio of truss girder span to the length of the single Vierendeel panel of 9.0, the relationship between the story drift and the plastic rotation of the special segment chord members is shown in Table 1. It can be seen that the plastic

4 rotational demand of the chord member in the special segment is as high as 17 rad at 2% story drift. Moreover, conventional STFs are prone to kinking in columns at the relatively flexible non-truss level under lateral loads (1) where θ = plastic rotation of the chord member = rotation of the chord member in the special segment with respect to the horizontal axis β = rotation of the vertical member near the special segment with respect to the vertical axis L = truss girder span length L = length of the single Vierendeel panel Figure 2. Deformed shape of a single truss girder. Table 1. Plastic rotation of chord members and story drift of a typical STF relationship Story Drift Ratio (%) Plastic Rotation (rad.) (a) (b) Figure 3. 3-D Models of STF systems: (a) conventional STF system; (b) modified STF system.

5 Proposed Modified Structural Layouts A prototype mid-rise STF building, called Base Model, was designed according to the spectrum specified in the 1997 NEHRP [1]. The structure layouts and geometry are shown in Figs. 3 and 4. Kickers were added at the non-truss levels to alleviate the demand on columns. The single Vierendeel panel was modified to span over three panels of the truss to reduce the rotational demands in the chord members, thereby increasing the overall drift capacity of the structure. Horizontal steel trusses, with conventional precast hollow-core slabs spanning across the transverse direction, towards the ends of the STFs were chosen as floor diaphragms to effectively transfer the in-plane shear to STFs through direct axial forces in the truss members. The horizontal trusses are shifted to avoid the direct gravity loading on the Vierendeel panels so that the yielding members are not resisting gravity loads. Pinned-connections were modeled between the horizontal trusses and STFs to minimize bending moment induced by the lateral displacement in the longitudinal direction. With the horizontal trusses acting as floor diaphragm, the precast slabs, which are simply supported on the horizontal truss, only transfer their own inertia forces; therefore, the demand at the slab-to-truss connection reduced drastically. Odd bay Even bay (a) (b) (c) (d) (e) Figure 4. Structural geometry of the 6-story STF prototype buildings with modified STF system: (a) plan view (horizontal trusses not shown for clarity); (b) floor diaphragms with horizontal trusses; (c) longitudinal side (moment frame) view; (d) odd bay; (e) even bay. The prototype buildings were designed and analyzed for their seismic responses through a series of nonlinear time-history analyses for both the design basis earthquake (DBE, 10% probability of exceedance in 50 years) and the maximum considered earthquake (MCE, 2% probability of exceedance in 50 years) level ground motions in both the transverse and the longitudinal directions.

6 Design Procedure The prototype steel buildings with 6-bay 6-story STF, as explained previously, was designed using a computer program, SAP2000 [2]. All members in the prototype building was elastically designed based on load combinations as defined in ASCE 7-10 [3] with redundancy factor and overstrength factor equal to 1.0. The structure was 64 ft 1½ in. by 180 ft in plan and 64.5 ft in height. The floor height on the first level was 12 ft where the upper levels are 10.5 ft. There were six bays with 36 ft spacing on centers in the transverse direction. The STFs serve as the seismic force-resisting system in the transverse direction. In the longitudinal direction, conventional moment-resisting frames were used along the perimeter as the seismic force resisting system. The design gravity loads for the model structures are similar to the loads used in AISC Steel Design Guide Series No.14 [4]. The dead load of 92 psf and live load of 40 psf were used as gravity load. Structural member design was carried out according to the load and resistance factor design (LRFD) of AISC [5] with ASTM A992 steel. Because the STF system is not addressed as a seismic force-resisting system in ASCE 7-10 Table [3], a response modification factor, similar to that of steel Special Concentrically Braced Frames (SCBF), of 6.0 was conservatively applied in the seismic design. Design of Members The 6-story STF building was designed in accordance with the LRFD of AISC [5]. Load combinations along with seismic load effects were as defined in ASCE 7-10 [3]. The top and bottom chords of the trusses were treated as continuous members. The web members were rigidly connected to the chord members. The chord members were assumed to be rigidly connected to the columns except where there was no diagonal web member connecting to the column at that location. In such case, typical construction details suggest that the chord members are more appropriately assumed to be pin-connected to the columns [6]. All of the horizontal truss members were considered pin-connected. Because the spandrel beams were part of the moment-resisting frames, they were treated as rigidly connected to the columns in their strong axis direction. Interior beams were considered pin-connected to the truss chords to minimize force transfer from the horizontal trusses to the STFs. The column sizes were kept within the typical W-shape columns. All of the members were selected such that all compression elements in the members were nonslender elements as defined in AISC [5]. In addition, width-tothickness ratios of the members were selected according to the requirement for highly ductile members specified in the AISC Seismic Provisions [7]. During the design process, a preliminary pushover analysis was also carried out to identify the plastic hinge locations of the expected yield members to ensure that the yielding of members is limited within the Vierendeel panels. This design approach is similar to that for the special segments with Vierendeel panels in the Special Truss Moment Frames (STMFs) [8], in which the yielding of members is limited within the Vierendeel panels of the STFs under large lateral forces. Analysis Modeling Nonlinear static pushover and time-history analyses for the 3-D model were carried out using a

7 computer program, Perform-3D [9]. All of the members except the diagonal web members in the STF trusses, hangers, posts, and the diagonal braces in the non-truss level were modeled as standard frame (beam-column type) elements. In addition, the truss members and columns were modeled considering axial-moment (P-M-M) interaction and with moment-rotation plastic hinges at both ends. The expected yield and ultimate strengths were determined by applying the material overstrength factor, and the strain-hardening adjustment factor. In this study, the respective values are considered to be 1.1 for both of them. The column maximum total rotational capacity was considered to be 0.07 rad. before strength degradations occur according to recent study [10]. Although the previous test results were for the W14 columns bending about the strong axis, it was used for both the strong and weak-axis bending for this study. Note that for wide-flange shapes, the yield surfaces for the axial-moment interaction are different in majoraxis flexure (used in the moment frames along the longitudinal direction of the building) and minor- axis flexure (used for the STFs along the transverse direction of the building). This was accounted for by using different yield surface parameters for the axial-moment interactions [11, 12]. The diagonal web members in the STF trusses, hangers, posts, and the diagonal braces in the non-truss level were modeled as buckling-type steel strut. The expected yield and ultimate strengths of those members were determined in accordance with brace members as per AISC Seismic Provisions [7]. The lengths of yielding or buckling segments of the diagonal web members and the diagonal braces were taken as 85% of their working point lengths to account for the gusset plates and rigid zones of columns or chord members at the ends of those members. This effective length, 0.85L, was used in the computation of the compressive strength of the buckling type members. P-Delta effect due to gravity loads was included in the analyses. Rayleigh damping (combination of the mass and stiffness proportion damping) and a damping ratio for welded steel structure of 2% were used throughout the analysis [13]. Force Transfer Pattern Analysis Results During the preliminary design process, it was observed that, when the diagonal braces were introduced to the STF system, the story shear forces were transferred in a more direct manner (i.e., most forces were transferred directly from upper truss to the lower non-truss and so forth) and no longer primarily depended on staggered manner via the diaphragms to transfer the lateral forces as in the conventional STF system. This significantly reduced the demands in the diaphragms as well as the connections between the diaphragms and STFs. The story shear distributions in each bay of the conventional STF system and the modified system when subjected to code-specified lateral forces from the elastically designed models are summarized in Table 2. The rigid floor diaphragm was modeled to represent the story shear transfer mechanism of the hollow-core slabs in the conventional STF model whereas the horizontal trusses were used as the shear diaphragm in the modified structural layouts of the STF.

8 Table 2. Story shear in individual bay (kips). Story Bay 1 Bay 2 Bay 3 Conventional Base Conventional Base Conventional Base * 43* * 77* * 66* * 128* * 104* * 72* * 260* * 145* 247* 236* 157* 161* Story Bay 4 Bay 5 Bay 6 Conventional Base Conventional Base Conventional Base 6 35* 45* * 49* * 64* * 117* * 123* * 68* * 250* * 194* 1 248* 222* 128* 139* 185* 192* *indicates non-truss bay Pushover Analysis Due to the structural irregularity of STF system, roof drifts of all bays were used during the pushover analysis as the reference drifts. The analysis would terminate when a drift ratio at roof elevation in any bay reached 2%. It was found that the bay on grid 1 (as shown in Fig. 4) reached 2% roof drift before other bays and was chosen as the controlling bay. Roof drift of this bay was then used as the control drift for the pushover analysis. Because roof drifts are varied from one bay to another, roof drift from bay 1 was also used as the roof drift limit during the design process. Fig. 5 shows how the roof drift was calculated during the design and analysis processes. It was found that the first yielding of the Base Figure 5. Roof drift calculation. and the Target Drift models are at rather small story drifts of about 0.3%, similar to that of a typical concentrically braced frame [7]. Nonlinear Time-History Analysis The 3-D model was analyzed to evaluate the seismic performance under forty SAC recorded ground motions representing DBE and MCE hazard levels in both the transverse and the longitudinal directions. The SAC ground motions have been scaled to match the 1997 NEHRP spectrum [14]. The main investigating parameter is the interstory drift ratio response in the direction of STFs.

9 The maximum interstory drift ratios (MIDR) of each ground motion are calculated using the greater of the absolute maximum or absolute minimum interstory drift ratio from all of the bays within each story. The average values of MIDR for each hazard level were also calculated. The results of the time-history analysis are shown in Fig. 6. Figure 6. Maximum interstory drift ratios: (a) DBE level ground motions (LA01 to LA20); (b) MCE level ground motions (LA21 to LA40). Note: * indicates the ground motions of which the analyses stopped before the end of the ground motion durations due to excessive damage of the yielded members The member colors indicate the minimum usage ratio (demand versus capacity of the member) as followed: Color Usage Ratio Grey 0.0 Teal 0.4 Green 0.6 Orange 0.8 Red Figure 7. Bay 1 deflected shape at the maximum roof drift with maximum plastic hinge rotations (%) and maximum interstory drift ratios (%) under LA16 ground motion. The average values of MIDR range from % and % for the DBE and

10 MCE hazard levels respectively. Comparing to the conventional STF system [15], the modified STF system gives relatively uniformed MIDR throughout the height of the structure. Figs. 7 and 8 show the maximum plastic hinge rotations and their locations and the maximum interstory drift ratios along with the minimum usage ratios of the members in the deflected shape of bay 1 at the maximum roof drift ratio under selected severe ground motions. It can be seen that only a few non-intended yielding members were subjected to yielding deformation even though they were not designed with the capacity design approach The member colors indicate the minimum usage ratio (demand versus capacity of the member) as followed: Color Usage Ratio Grey 0.0 Teal 0.4 Green 0.6 Orange 0.8 Red 1.0 Figure 8. Bay 1 deflected shape at the maximum roof drift with maximum plastic hinge rotations (%) and maximum interstory drift ratios (%) under LA36 ground motion. Conclusions A modified STF system in which the diagonal braces were added in the non-truss frames to alleviate the demand on columns along with horizontal trusses serving as in-plane shear diaphragm is proposed to improve the seismic response of the STF system. The preferred yield mechanism of the STF system is where plastic hinges form only within Vierendeel panels located at the center of the truss similar to that in the special segments of the Special Truss Moment Frames (STMFs). A single Vierendeel panel in the middle of truss girder leads to extremely high rotational demands in the chord members even at small roof drift ratios. In order to increase the overall drift capacity of the structure, the Vierendeel panels were expanded over three panels. This reduced the rotational demand at the ends of the chord members in the truss and allowed the structure to reach larger drifts. The addition of kickers altered the seismic load transfer path from a staggered pattern via floor diaphragms to a more direct path from upper truss to the non-truss at lower level. This, in turn, relived the high force demands in the diaphragms and the diaphragm-to-truss connections. Analysis results showed that multiple Vierendeel panels combined with kickers can effectively reduce the demands in the non-truss stories. It can be seen from the analysis results that STFs with the modified configurations showed stable

11 responses under severe ground motions. Nonlinear time-history analyses indicated that the modified STF could potentially be used in seismically active areas. Acknowledgments This research project was sponsored by National Science Foundation (Grant No. CMMI , NEESR-CR: Steel Truss Systems with Enhanced Seismic Safety and Performance) and American Institute of Steel Construction. The opinions and views expressed in this paper are solely those of the authors and do not reflect those of the sponsor. References 1. Federal Emergency Management Agency (FEMA). NEHRP recommended provisions for seismic regulations for new buildings and other structures (FEMA 450), Part 1: Provisions. Washington, D.C., Computers and Structures, Inc. (CSi). CSI analysis reference manual for SAP2000, ETABS, SAFE and CSiBridge. CSi: Berkeley, California, American Society of Civil Engineering (ASCE). Minimum design loads for buildings and other structures. ASCE: Reston, Virginia, Wexler N, Lin F-B. Steel Design Guide Series No. 14 Staggered truss framing systems. AISC: Chicago, Illinois, American Institute of Steel Construction (AISC). Specification for structural steel buildings. AISC: Chicago, Illinois, Marstellar B, Faraone T. Anatomy of a staggered truss. Modern Steel Construction. 2002; 42 (9): American Institute of Steel Construction (AISC). Seismic provisions for structural steel buildings. AISC: Chicago, Illinois, Chao S-H, Goel SC. Performance-based plastic design of special truss moment frames. Engineering Journal 2008; 45 (2): Computers and Structures, Inc. (CSi). Components and elements for PERFORM-3D and PERFORM- COLLAPS. CSi: Berkeley, California, Newell JD, Uang C-M. Cyclic behavior of steel wide-flange columns subjected to large drift. Journal of Structural Engineering 2008; 134 (8): El-Tawil S, Deierlein GG. Nonlinear analysis of mixed steel-concrete frames. I: Element formulation. Journal of Structural Engineering 2001; 127 (6): El-Tawil S, Deierlein GG. Nonlinear analysis of mixed steel-concrete frames. II: Implementation and verification. Journal of Structural Engineering 2001; 127 (6): Chopra AK. Dynamics of structures Theory and applications to earthquake engineering, 4 th Ed. Pearson Prentice Hall, New Jersey, Somerville PG, Smith NF, Punyamurthula S, Sun JI. Development of ground motion time histories for phrase 2 of the FEMA/SAC steel project. Rep. No. SAC/BD-97/04. SAS joint Venture, Sacramento, California, Kim J, Lee J-H, Kim Y-M. Inelastic behavior of staggered truss systems. The Structural Design of Tall and Special Buildings 2007; 16: